Thermoplastic and thermoset polymeric materials have been widely used in the construction of inflatable bladders. Inflatable bladders have been used in a variety of products such as vehicle tires, balls, accumulators used on heavy machinery, and in footwear, especially shoes, such as for cushioning devices. The bladders may be made of elastic materials, or elastomers, which are able to substantially recover their original shape and size after removal of a deforming force, even when the part has undergone significant deformation.
Footwear, and in particular shoes, usually include two major components, a shoe upper and a sole. The general purpose of the shoe upper is to enclose the foot. For example, the shoe upper may be made from an attractive, highly durable, comfortable material or combination of materials. The sole, constructed from a durable material, is designed to provide traction and to protect the foot during use. The sole also typically serves the important function of providing enhanced cushioning and shock absorption during athletic activities to protect the feet, ankles, and legs of the wearer from the considerable forces generated. For example, during running, the force of impact generated can amount to two or three times the body weight of the wearer, while other athletic activities such as playing basketball may generate forces of between six and ten times the body weight of the wearer. Many shoes, particularly athletic shoes, now include some type of resilient, shock-absorbent material or components to cushion the foot and body during strenuous athletic activity. These resilient, shock-absorbent materials or components are commonly referred to in the shoe manufacturing industry as the midsole. Such resilient, shock-absorbent materials or components can also be applied to the insole portion of the shoe, which is generally defined as that portion of the shoe upper directly underlying the plantar surface of the foot.
Gas-filled bladders may be used for midsoles or inserts within the soles of shoes. The gas-filled bladders are generally inflated to significant pressures in order to cushion against the forces generated on the foot during strenuous athletic activities. Such bladders typically fall into two broad categories, those that are “permanently” inflated, such as disclosed in Rudy, U.S. Pat. Nos. 4,183,156 and 4,219,945, and those using a pump and valve system, such as those disclosed in, U.S. Pat. No. 7,051,456 and US Publications 2005/0132606, 2005/0022422, and 2002/0194747.
Athletic shoes of the type disclosed in U.S. Pat. No. 4,183,156 having “permanently” inflated bladders have been sold under the trademark “Air-Sole” and other trademarks by Nike, Inc. of Beaverton, Oreg. Permanently inflated bladders of such shoes are constructed using an elastomeric thermoplastic material that is inflated with a large molecule gas that has a low solubility coefficient, referred to in the industry as a “super gas.” Permanent inflation also may be achieved with less expensive gasses like air or nitrogen. For example, U.S. Pat. Nos. 5,083,361 and 5,543,194 discloses selectively permeable sheets of film that are formed into a bladder and inflated with a gas or mixture of gases to a prescribed pressure.
The industry has moved away from using “super gas” because of the environmental impacts and the negative public image associated with it. For example, bladders in newer shoes are often filled with nitrogen. As a result, the bladders may be made with either multi-layer or micro-layer films. Examples of patents describing such air bladders include, but are not limited to U.S. Pat. Nos. 6,013,340; 6,082,025; 6,203,868; 6,391,405; 6,599,597; and 7,730,379, hereby incorporated by reference in their entirety.
Historically, colored inflatable bladders (such as airbags) have been produced by painting or from colored sheets, both of which involve the use of pigments. Painting results in uneven application and can be opaque in appearance. Painting can be a slow process and labor intensive. The use of colored sheets results in waste from colored trim scrap which has to be recycled for economic reasons.
Dyes are typically used in clear plastics to provide bright color and transparent shades where optical clarity is important. Examples include polycarbonate, polystyrene, PET and acrylics. Dyes are selected based on the solubility: they have to be soluble in these polymers. In other words, they are dispersed in the polymer matrix on the molecular level and therefore no scattering of the light occurs.
One inherent difficulty of coloring polymers with dyes is dye migration. Some dyes form covalent bonds with dyed polymers to prevent bleeding. However, most dyes lack the ability of forming such bond, hence have only poor to fair colorfastness in thermoplastics. Intermolecular interaction of the dyes with the substrates becomes important to minimize dye migration. The types of forces that may govern the interactions include, in the order of decreasing strength, electrostatic forces, hydrogen bonds and Van der Waals forces.
Over time, dyes will migrate from the substrate to the surface because of the mobility of dyes. To reduce the migration, the use of dyes is often limited to those polymers that have relatively high glass transition temperature (Tg). For example, the Tg of polycarbonate, polystyrene and PET are 145° C., 100° C. and 70° C., respectively, which are significantly higher than the typical temperatures that the polymers are being used. So the dye molecules are “frozen” in the polymers.
Thermoplastic elastomers such as polyurethane and polyether block amides are soft materials at ambient temperatures. Their Tg generally are much lower and therefore it is more difficult to prevent dye migration in those polymers.